For instance, a known seat reclining device is disclosed in U. S. patent published as U.S. Pat. No. 6,244,661. In the seat reclining device, a pair of reclining mechanisms including locking mechanisms is provided at the right and left sides of a seat device respectively. When a vehicle crashes into an object located in the front of the vehicle, the seat back is pulled toward the front side of the seat device by the seat belt connected to the upper end of the seat back, and then receives load. The pair of reclining mechanisms has enough structural strength against the load. And the reclining mechanisms are connected each other via a connecting mechanism and operated together.
However, in the above-disclosed seat reclining device, torsional deformation is generated on the seat back as illustrated with chain double-dashed line in FIG. 8, because the seat back receives the load from the seat belt at the either right or left side of the upper end of the seat back. Therefore, a reclining mechanism 51a is rotated in counterclockwise direction (in an arrowed direction shown in FIG. 8 relative to a reclining mechanism 51b. Under this condition, the operation handle 52 is engaged with a connecting mechanism 80, movement of the operation handle 52 is restricted by the reclining mechanism 51b, therefore the reclining mechanism 51a is rotated in counterclockwise direction relative to the operation handle 52. Thus, the operation handle 52 is rotated in clockwise direction to the reclining mechanism 51a. And then, the locking mechanism of the reclining mechanism 51a is unlocked, an occupant cannot be sufficiently protected by the seat back.
A need thus exists to provide a seat reclining device in which a locking mechanism is not unlocked when torsional deformation is generated on the seat back, and which is capable of protecting an occupant by efficiently absorbing energy of crash with the occupant being restrained by the seat back.